1. Field of the Invention
The present invention relates to a permanent-magnet electric generator using permanent magnets as its field system, and an electricity generation device provided with the permanent-magnet electric generator.
2. Description of the Related Art
As such permanent-magnet electric generators using permanent magnets as their field system, a three-phase alternating-current (AC) generator has been known, for example. In the three-phase AC generator using permanent magnets, a rotor constituted by the permanent magnets is rotated inside a stator constituted by a three-phase winding, for example, to generate three-phase AC power in accordance with the law of electromagnetic induction. Compared with an electromagnet electric generator using electromagnets as its field system, the permanent-magnet electric generator has the advantages of simpler structure, easier maintenance and the like, because the permanent-magnet electric generator does not require a power circuit for field excitation and also does not have electrical contacts, such as a slip ring or brush, for guiding electric current from an external power circuit to the rotating field.
Meanwhile, conventional permanent-magnet electric generators do not stop generating electricity unless and until the rotor is stopped, even if an electricity generation device equipped with the electric generator develops some trouble or failure. If a circuit breaker is provided between the output of the permanent-magnet electric generator and a load, for example, the supply of electric power from the permanent-magnet electric generator to the load can be shut off in the event some trouble or failure has occurred in the electricity generation device. As conventional techniques for shutting off or restraining the power supply from a permanent-magnet electric generator to a load, a permanent-magnet electric generator with a clutch mechanism capable of interrupting the transmission of rotary driving force to the rotor has been known, for example (see Japanese Laid-open Patent Publication No. 2005-261008). There has also been known a rotary electric machine which includes a rotor provided with field windings in addition to permanent magnets and which is capable of stopping the output of an output current by controlling the field current flowing through the field windings (see Japanese Laid-open Patent Publication No. 2005-204482). Further, a permanent-magnet electric generator has been known which is provided with a regulator (voltage regulation device) capable of controlling the output voltage in accordance with load condition (see Japanese Laid-open Patent Publication No. 2003-189697).
According to the conventional technique disclosed in Japanese Laid-open Patent Publication No. 2005-261008, however, the electricity generation device is complex in structure and large in size because of the clutch mechanism provided therein, which possibly leads to a significant increase in cost. In the conventional techniques disclosed in Japanese Laid-open Patent Publications No. 2005-204482 and No. 2003-189697, on the other hand, if phase-to-phase short circuit, for example, occurs within the permanent-magnet electric generator, the short-circuit current cannot be stopped, and therefore, it is not possible to prevent the permanent-magnet electric generator from suffering burnout or the like due to the phase-to-phase short circuit. This also applies, for example, to the case where the electricity generation device is provided with a circuit breaker for shutting off the output of the permanent-magnet electric generator. Further, the conventional techniques disclosed in the aforementioned publications are associated with the problem that, since the output voltages of the permanent-magnet electric generator are shut off or restrained in accordance with an externally monitored condition, a certain length of time is required to shut off or restrain the output voltages after the occurrence of anomaly or failure in the permanent-magnet electric generator.